1. Field of the Invention
The present invention relates to a zoom lens system suitable for a single lens reflex (SLR) camera, and in particular, relates to a small zoom lens system which has magnification ratio (zoom ratio) of approximately 3.0, and includes a four-lens-group arrangement, i.e., a negative lens group, a positive lens group, a negative lens group, and a positive lens group, in this order from the object.
2. Description of the Prior Art
In the prior art, in a negative-lead type zoom lens system is frequently used in a zoom lens system which can easily make the focal length at the short focal length extremity shorter, and can achieve the photographing angle-of-view of 70xc2x0 or more. In recent years, a standard zoom lens system used in a SLR camera and the like have been required to have the shorter focal length at the short focal length extremity, and to be smaller in size, and to be inexpensive in cost.
The present invention achieves a zoom lens system of a four-lens-group arrangement, i.e., a negative lens group, a positive lens group, a negative lens group, and a positive lens group, in this order from the object. The zoom lens system of the present invention can have the shorter focal length at the short focal length extremity, and can be made smaller in size, and inexpensive in cost, by appropriately determining the refractive powers, the lens arrangement, and movement (traveling) conditions, etc., of the each lens group.
As an aspect of the present invention, there is provided a zoom lens system including a negative first lens group, a positive second lens group, a negative third lens group, and a positive fourth lens group, in this order from the object.
Upon zooming from the short focal length extremity to the long focal length extremity, the distance between the negative first lens group and the positive second lens group decreases, the distance between positive second lens group and the negative third lens group increases, and the distance between the negative third lens group and the positive fourth lens group decreases.
The negative first lens group includes a negative meniscus lens element having the convex surface facing toward the object, a negative lens element, and a positive meniscus lens element having the convex surface facing toward the object, in this order from the object.
The negative third lens group includes a positive lens element and a negative lens element which are either cemented to each other or separated from each other, in this order from the object.
The positive fourth lens group includes a positive lens element and a negative lens element which are either cemented to each other or separated from each other, in this order from the object.
The zoom lens system satisfies the following conditions:                     0.25         less than                   "LeftBracketingBar"                                                    f                3                            /              f                        ⁢                          xe2x80x83                        ⁢            t                    "RightBracketingBar"                 less than         0.5                            (        1        )                                0.07         less than                               log            ⁡                          (                              Z                34                            )                                /                      log            ⁡                          (              Z              )                                       less than         0.17                            (        2        )                                          -          0.17                 less than                                             r                              L                -                1                                      /            f                    ⁢                      xe2x80x83                    ⁢          t                 less than                   -          0.14                                    (        3        )            
wherein
f3 designates the focal length of the negative third lens group;
ft designates the focal length of the entire zoom lens system at the long focal length extremity;
m34T designates the combined magnification of the negative third lens group and the positive fourth lens group at the long focal length extremity;
m34W designates the combined magnification of the negative third lens group and the positive fourth lens group at the short focal length extremity;
Z34=m34T/m34W;
Z designates the zoom ratio ((focal length at the long focal length extremity)/(focal length at the short focal length extremity)) of the entire the zoom lens system; and
rLxe2x88x921 designates the radius of curvature of the object-side surface of the negative lens element of the positive fourth lens group.
Furthermore, the positive second lens group can include a positive lens element, and cemented lens elements having a positive lens element and a negative lens element, in this order from the object.
The present disclosure relates to subject matter contained in Japanese Patent Application No. 2001-331086 (filed on Oct. 29, 2001) which is expressly incorporated herein in its entirety.